**5. Construction steps**

### **5.1 Facility layout**

The anaerobic digestion can be accomplished in one digester, and thus the facility is called 'single-stage biogas facility'. In other facility layouts, the anaerobic digestion can be carried out in two stages, i.e. in two different tanks in order to optimize the operating conditions, and thus the facility is called 'two-stage biogas facility'. The single-stage facility is a simple design with a longer track record, and has lower capital costs and technical problems. The two-stage facility has shorter retention time as each stage design is optimized. There is a potentially higher biogas production from two-stage facilities, but they require higher capital costs.

Subsequent to the site investigations such as the soil specifications ground water level, the facility layout should be planned. The commercial biogas plants consists of a fermenter and a secondary fermenter or so called "follow-up fermenter", where both have identical dimensions, usually as follows: height of 6 m, internal diameter of 23 m, and external diameter of 23.70 m. This implies that the thickness of the fermenter wall is usually 35 cm. A residue storage tank is annexed to the fermenters, where the tank has an internal diameter of 25 m, external diameter of 25.70 m and a height of 6 m (Fig. 4). The solids feeder is located adjacent to the fermenter, and the tanks are surrounded by green belt from all sides except one side where the horizontal silos are located.

Table 2. Required quantities of construction materials to build the digester

4Consider digester load of 4 kg m3 day-1 and density of 1.2 kg m-3

cow-1 month-1, and 100 cows

**5. Construction steps** 

**5.1 Facility layout** 

capital costs.

Table 3. Quantities of the substrates

one side where the horizontal silos are located.

of 11820 m3

Material Quantity Unit Rebar 36 Ton Cement 320 Ton Sand 400 m3 Gravels 800 m3

Material Quantity Unit Raw slurry storage1 18 m3 Liquid organic matter2 21 m3 Liquid substrate3 80 m3 Dry organic matter4 267 m3 day-1 Total substrates5 10750 m3 1Consider a duration of 3 days for mixing and pumping, daily manure deposition of 6 m3 day-1, 1.8 m3

2Consider a storage duration of 7 days and liquid organic matter deposition of 3 m3 day-1 3Consider 40 days of storage duration and liquid substrate deposition of 2 m3 day-1

5Total quantity of substrates (10750 m3) that should be stored in a digester having a capacity

The anaerobic digestion can be accomplished in one digester, and thus the facility is called 'single-stage biogas facility'. In other facility layouts, the anaerobic digestion can be carried out in two stages, i.e. in two different tanks in order to optimize the operating conditions, and thus the facility is called 'two-stage biogas facility'. The single-stage facility is a simple design with a longer track record, and has lower capital costs and technical problems. The two-stage facility has shorter retention time as each stage design is optimized. There is a potentially higher biogas production from two-stage facilities, but they require higher

Subsequent to the site investigations such as the soil specifications ground water level, the facility layout should be planned. The commercial biogas plants consists of a fermenter and a secondary fermenter or so called "follow-up fermenter", where both have identical dimensions, usually as follows: height of 6 m, internal diameter of 23 m, and external diameter of 23.70 m. This implies that the thickness of the fermenter wall is usually 35 cm. A residue storage tank is annexed to the fermenters, where the tank has an internal diameter of 25 m, external diameter of 25.70 m and a height of 6 m (Fig. 4). The solids feeder is located adjacent to the fermenter, and the tanks are surrounded by green belt from all sides except

(a) MT-ENERGIE GmbH & Co. KG

(b) General process scheme of the two-stage anaerobic digestion process (Blumensaat and Keller, 2005)

Fig. 4. Facility layout for a two-stage biogas plant

Biogas Plant Constructions 353

The bottom of the pit should be concave, where the center of the digester should be the most concave. Generally, the pit shape depends on the design of the digester, where in the case of round-shaped household units (Figs. 6a and b) a guide wood post is installed in the center of pit bottom. A string is linked to the post and used to set the round-shape of the pit. On the other hand, in the case of commercial biogas plants (Fig. 7), total station, teodolit or laser leveling is used for surveying. For large digesters, i.e. for commercial biogas plants,

The pit's bottom should be cleaned, and the gridiron is built (Fig. 8) using a pre-selected type of iron rods as either 6Ø6 m-1 or 6Ø8 m-1. Subsequently, the concrete mixture is poured (Fig. 9 and 10). The water:cement ratio is 0.53 L kg-1 and the cement:sand:gravel mass ratio is 1:2.2:3.7. The thickness of the concrete base ranges between 10-25 cm depending on the soil's

Fig. 8. Structuring the grid irons for a commercial biogas plant (MT-ENERGIE GmbH & Co. KG)

bulldozers are used to achieve the excavation (Fig. 7).

Fig. 7. Excavation works for a commercial biogas plant

**5.4 Preparation of the digester's bottom** 

specifications and the ground water level.
